Centrifugal pump for operating at low suction head

ABSTRACT

A CENTRIFUGAL PUMP IS DESCRIBED HAVING AN AXIAL RAMP STRUCTURE AT THE INLET WHICH ALLOWS HOT LIQUIDS TO BE PUMPED AT VERY LOW INLET PRESSURE WITHOUT LOSS OF FLOW OR CAPACITY DUE TO CAVIATION.

July 4, 1972 J, w. ERICKSQN 3,674,378

CENTRIFUGAL PUMP FOR OPERATING AT LOW SUCTION HEAD Filed Aug. 13, 1970 2 Sheecs$heet 2 mes F|G.6 FIG.7 F|G.8

124 124 Ala "United States Patent Office Patented July 4, 1972 US. Cl. 416-177 6 Claims ABSTRACT OF THE DISCLOSURE A centrifugal pump is described having an axial ramp structure at the inlet which allows hot liquids to be pumped at very low inlet pressures without loss of flow or capacity due to cavitation.

This application is a continuation-in-part of the application filed on Sept. 23, 1968, Ser. No. 761,769, under the title Liquid Cooled Electric Motor, now Pat. No. 3,525,- 001, issued Aug. 18, 1970.

The invention described and claimed in the parent application has to do generally with the cooling of dynamoelectric machines such as motors by circulation of liquid in heat exchanging relation with their stator and rotor assemblies.

That invention is particularly useful in the cooling of electric motors used for driving liquid pumps, as for pressurizing hydraulic fluid and the like.

In accordance with one aspect of that invention, the entire liquid flow to the main pump is caused to pass in heat exchanging relation through passages suitably formed in the motor, thus removing heat at the required rate with relatively little elevation of the temperature of the circulated liquid. Vaporization due to local heating is thereby brought under control.

A further aspect of that invention provides a forepump integrated in the motor structure for pressurizing the input liquid prior to circulation through the motor and prior to delivery to the main pump.

The present invention provides pump structure for reducing any tendency to caviation even at very low inlet pressures. For that purpose the impeller is designed with an axial ramp and very low angle of lift at the inlet. The net positive suction head is further reduced by supplying liquid radially to the impeller inlet and providing a sharp cutofi at the inlet.

Further objects and advantages of the invention will be understood from the following description, with reference to the accompanying drawings, in which:

FIG. 1 is a fragmentary axial section on line 11 of FIG. 2, representing an illustrative embodiment of the invention;

FIG. 2 is a section, generally on line 2-2 of FIG. 1 and partially broken away;

FIG. 3 is an elevation of the impeller in aspect opposite to FIG. 2; and

'FIGS. 4 through 8 are axial sections on the corresponding lines of FIG. 3.

Since the present application is concerned mainly with the pump structure, the motor with which the present illustrative pump is associated is shown in this application only in fragmentary form. A full description of the motor will be found in the above identified parent application.

The present motor is specifically designed to drive a main pump for supplying a liquid, typically a hydraulic fluid, at high pressure, as for operating hydraulic equipment on an aircraft, for example. Such a pump may be of any suitable type and is therefore indicated only schematically at 34 in FIG. 1, with pump inlet at 35 and pump outlet for pressurized liquid at 36. The pump housing typically includes an end wall 37 with flange structure adapted to engage positively the inner end of motor housing 20, to which it is connected by the bolts '39 (FIG. 2) with suitable fluid seal 38. The pump shaft, indicated at 56 is journaled on the bearing 57, carried by housing wall 37, and on one or more other bearings, not shown. Pump shaft 56 is coupled to motor shaft 40 by the coupling element 58, which typically has splined engagement with both the shafts. Liquid at low pressure is supplied to the motor-- pump assembly via the inlet conduit 54 and the fitting 55 in the motor housing. Before reaching the pump inlet, the entire body of that liquid is employed for cooling the motOI.

The motor proper comprises the rotor 48 and the stator 45. The rotor laminations 42 are fixedly mounted on the central section 44 of motor shaft 40 within motor chamber 30, and carry the rotor windings 43. The stator laminations 46 are rigidly mounted within the stator shell 50, with stator windings 47. Motor chamber 30 is sealed at shaft bearing 31, where the motor shaft passes through the end wall 25, by the live seal indicated at 65, which may be of conventional construction. Stator shell 50 is spaced inwardly from housing shell 22, defining between them an annular cooling passage structure 60. Seals are provided at 63 between stator shell 50 and the flange 62 of the end casting 24, and at 64 between housing shell 22 and the outer periphery of the end casting.

Shaft 40 is preferably constructed as shown, with a central portion 44 of relatively large diameter and small wall thickness, and with a smaller end section at bearing 31. Shaft section 44, on which rotor laminations 42 are mounted, is provided with the heat conducting fins 82.

Inner end casting 24 of the motor housing includes the generally cylindrical shell portion 33 which extends axially from end wall 25 and carries main pump 34. Shell 33, end wall 25 and pump end wall 37 define the chamber 100. At least the end of that chamber adjacent the motor is cylindrical in form and receives the forepump impeller, designated generally by the numeral 120. The impeller is rigidly mounted on the end of motor shaft 40 by the retaining nut 123 and the defining ball 121. Impeller comprises the hub 122 and the circular plate 124 which carries on its axially outer face the spirally formed pump vanes 126. Impeller 120 also includes the annular cover plate 128, typically cast integrally with vanes 126 and defining with those vanes and plate 124 the intervane passages 138 through which liquid is pumped centrifugally by pump rotation. Liquid enters those passages at their radially inner ends through the pump orifice 129 between hub 122 and annular cover plate 128 and leaves at the periphery of the impeller.

Plate 124 and cover 128 of impeller 120 fit with minimum clearance within chamber 100, so that the impeller efiectively divides that chamber into the inlet chamber 130 and the outlet chamber 140. Liquid is supplied to inlet chamber 130 from conduit 54 via the generally radial passage 134, formed integrally in end casting 124. The outer periphery of chamber 130 is somewhat enlarged at 132 to receive and distribute that entering liquid, which then flows inward between the radial directing vanes 136 and arrives at pump orifice 129 with little or no circumferential velocity.

Liquid is delivered from pump passages 138 at elevated pressure and is received in the annual channel 144. It is delivered from that channel through multiple passages in casting 24 to the axially inner end of annular passage structure 60, already described. After absorbing heat from the stator, the liquid flows through passages in the outer end casting to the outer end of the shaft and thence axially within shaft 40 the entire length of that shaft. Liquid is delivered from the inner end of shaft 40 through a bore 105 in coupling 54 and thence through radial apertures 106 to outlet chamber 140. Chamber 140 communicates via the passage 152 with the outlet fitting 154 and the conduit 156, which delivers the liquid directly to inlet 35 of main pump 34. After pressurization in pump 34 the liquid is supplied from pump outlet 36 via the conduit 158 to any desired device for utilizing the pressurized fluid. If the present motor structure is used with a main pump having its inlet and mounting structure at the same end, conduit 156 may be omitted, and the liquid delivered directly from chamber 140 through pump end wall 37 to the pump inlet. The bypassage 110 in shaft 40 returns a small fraction such as of the pressurized liquid to the forepump inlet, insuring active circulation even if the main pump flow is temporarily interrupted.

An important aspect of the present invention concerns the capability of the forepump to handle hot volatile liquids at very low values of the net positive suction head, such as may occur, for example, in aircraft at high altitudes in case of failure of the normal provision for pressurization. That capability is largely due to specific design features of the impeller, together with the previously described radial vanes 136 or their equivalent in inlet chamber 130.

FIG. 3 shows an illustrative impeller in partial axial elevation as seen from the inlet side, with annular inlet aperture 129 between hub 122 and the inner edge 177 of cover plate 128. Main impeller vanes are shown at 126, with secondary vanes at 126a. FIGS. 4 to 8 are sections taken in respective axial planes that differ progressively by 15 degrees, as indicated by the correspondingly numbered section lines in FIG. 3. Since there are four main and four secondary vanes in the present structure, uniformly spaced, the omitted half of each section is the same as that shown. The outer peripheries of impeller plates 1'24 and 128 are beveled at 167 and 168, providing essentially line seals with the cylindrical Wall 169 of the pump chamber. Liquid delivered radially outward from impeller passages 138 is therefore expelled through chamber 144 into conduits 145 (FIG. 4). It is noted, however, that the seal at plate 124 may be omitted if the chamber to the right of the impeller, as seen in the present structure, is not used as final delivery chamber from the motor housing.

The present impeller includes structure, omitted for clarity in FIG. 1, at the impeller eye, that is, inward of cover plate 128, forming the annular surface 174 which virtually fills inlet aperture 129 and which may be considered an outward extension of hub 122. The spiral channels 176 are formed in that surface. At their inner or entrance ends at 175 adjacent hub 122, those channels are shallow and narrow, as shown at 176a in FIG. 4. Fifteen degrees later in FIG. 5 the channel 176b is deeper and wider, but with its radially inner side wall still at the periphery of the hub. In FIG. 6 at 176a the channel is again deeper and wider, and has shifted radially outward, with its outer side wall about half way across annular surface 174. That trend continues through FIG. 7 until the radially outer channel sidewall moves under the sharp edge 177 of cover plate 128, merging smoothly into a vane 126. At FIG. 8 channel 1762 is still laterally open to suction over about half its width, the other half being covered by plate 128 in the transition into a fully enclosed passage 138. At FIG. 8 the channel depth already corresponds to the full axial dimension of main impeller passages 138, and a new channel is about to start. If the successive channels follow each other more closely than in the present struc ture, annular surface 174 may virtually disappear, being reduced to narrow edges between adjacent channels.

The gradual inclination of the channel bottom forms an axial ramp which controls the axial component of the motion of entering liquid. That ramp is inclined with re spect to annular surface 174 at an angle less than 30 and typically about 10. The radial lift angle of channels 176 is correspondingly small, typically of the order of 5 to 10, increasing gradually outward to provide smooth merging into the sidewalls of the regular impeller vanes. The angle of those vanes at entrance, that is, the angle between the vane face and the tangential direction at cover edge 177, is made as small as feasible, typically between 10 and 15 and in any case less than 20". Once the passages are fully enclosed, their direction departs progressively from the tangential direction, reaching an angle of the order of 30 to 40 at the impeller periphery, or from 2 to 3 times the entrance angle.

In the present structure, the four similar channels are distributed around the impeller axis at intervals of Each channel extends through an angle of that same order before it is fully covered by edge 177. The channel length, before it becomes gradually transformed into an enclosed passage between regular impeller blades, exceeds its maximum width by a factor that is at least about two, and that is about live in the present illustrative structure. For an impeller with only two main passages, that factor may be as large as 20.

The effect of the described structure at the impeller eye is to cause liquid from outside the impeller to enter the gradually forming channel without any abrupt expansive dynamic forces. Liquid entering a conventional pump encounters the leading edges of the vanes at an appreciable angle of attack, experiencing correspondingly vigorous dynamic forces. In the present structure, liquid enters channels 176 laterally, rather than longitudinally, and at a low relative velocity controlled by the gentle axial ramp at the channel bottom. Further, the nearly tangential direction of the channels corresponds generally to a low angle of attack. Thus, liquid is received into the channels with substantially zero dynamic depression. Moreover, the liquid enters the steadily deepening and widening channels with essentially zero initial forward rotation, due to the restraining action of radial vanes 136 in entrance chamber 130. If the pump housing permits axial approach of entering liquid, those vanes are modified accordingly.

Once in the channels, the liquid is subject to strong frictional force from the channel walls of the rotating impeller. That friction accelerates the liquid in a generally tangential direction, increasing its energy without dynamic action. The pressure is thereby increased virutally without any tendency to cavitation. The sharp cutoff at 177 has been found effective in further reducing the required net positive suction head, perhaps by cleanly separating the energized channel liquid from the ambient chamber liquid. With increasing spinning movement of the liquid, centrifugal force generates further fluid pressure increase, allowing a gradual increase in blade lift and leading to normal pump action as the radius increases. The described'impeller structure is thus able to pump hot liquids at a net positive suction head in the range of l to 3 feet without loss of capacity from cavitation.

In the present combination, the specially designed forepump permits input liquid to be received at pressures typically as low as 2 p.s.i.a., and delivers that liquid, after cooling of the motor, to the main pump at inlet pressures of several atmospheres. The main pump can therefore be designed for high performance without special attention to control of cavitation, and the combined motor and pump can operate at pressures below the range of conventional practice.

The embodiments that have been described are intended only as illustration of the invention, and many modifications may be made in those embodiments without departing from the proper scope of the invention, which is defined in the appended claims.

In particular, the portion of impeller described above as cover plate 128 may be considered to form part of the housing, with minimum clearance between its inner face and the individual impeller vanes '126. The sharp edge 177 is then preferably retained as part of the housing, its described action being essentially independent of whether it is fixed or rotates with the impeller. Thus, the invention is directly applicable to the so-called shroudless, as well as shrouded, impellers.

It is well known that when liquids containing dissolved or finely divided gas are pumped at low inlet pressure the gas tends to be liberated or to expand, and can cause a serious drop in capacity. The same characteristics of the present pump structure that make it elfective under conditions tending to produce cavitation also make it particularly useful for pumping liquids containing gas.

I claim:

1 A radial centrifugal pump impeller capable of operation at low net positive suction head, comprising in combination with a supporting hub radially outer structure at least partially defining a plurality of angularly spaced, laterally enclosed, spiral passages extending at angles with the tangential di rection that increase progressively along the passages outwardly from an entrance angle at the inner, suction end to a discharge angle at the impeller periphery,

and radially inner entrance structure forming a plurality of channels that extend predominantly tangentially and are open laterally on one axial side to suction, the channels increasing in axial depth progressively and smoothly from essentially zero at their entrance ends and merging smoothly at their discharge ends into respective ones of said passages,

whereby axially directed entering liquid enters the channels predominantly laterally and essentially without dynamic depression, and receives forward rotational energy by virtue of shear at the channel walls before entering said passages.

2. An impeller as defined in claim 1, and in which said entrance angle is less than about 15 degrees, and said discharge angle is from 2 to 3 times the entrance angle.

3. An impeller as defined in claim 1, and in which said radially inner structure includes a coaxial, generally annular surface in which the channels are formed.

4. An impeller as defined in claim 3, and in which said entrance angle is less than about 15 degrees, and

the angles between the lengths of the channel bottoms and said generally annular surface are less than said entrance angle.

5. An impeller as defined in claim 3', and in which said radially outer structure includes an annular cover having a shar inner peripheral edge that is essentially in the plane of said annular surface.

6. An impeller as defined in claim 1, and in which said radially outer structure includes a sharp circular inner edge forming the boundary between each channel and the passage into which it merges.

References Cited UNITED STATES PATENTS 867,069 9/ 1907 Neumann 416-186 1,238,731 9/1917 Anderson 415-106 2,309,327 1/1943 Naegele et al. 416-186 2,784,936 3/1957 Schmidt 416-186 3,261,297 7/1966 Daniel 415-213 HENRY F. RADUAZO, Primary Examiner US. Cl. X.R. 

